Rural communities around the world are in need of broadband services as is frequently commented on by governments and news media. The efforts to supply such services usually rely on setting up towers and transmitting at low power and at a high frequency range of well over 900 MHz. These systems are expensive and not economically feasible for areas with low population densities.
Recently, systems employing the 500-900 MHz range and operating in licensed UHF frequencies have been deployed in Canada to service First Nation Communities as well as rural communities. Such systems also require a tower. But the systems permit coverage of 2000 to 3000, or more, square kilometers rather than the 75-100 square kilometers covered by the systems using higher frequencies and lower power levels.
The system at the tower is in communication with a plurality of customers each of which has customer premises equipment (CPE). In a full duplex broadband system where high power signals are transmitted from a base station (the tower) to a plurality of customers premises equipment, these transmit signals, when received by customer premises equipment, have to be isolated from high power customer transmit signals. The isolation is accomplished traditionally by a duplexer. But in a full duplex broadband system, each customer premises equipment is assigned a different pair of transmit and received channel frequencies. Consequently, a duplexer of different specification is required for each customer.
The duplexer employs two multistage filters and is operative to allow bi-directional communication over a single antenna. For CPE, it operates to separate the receive signal from the transmit signal while permitting them to share a common antenna. The duplexer must be designed for operation in the frequency band (470-860 MHz) used by the receiver and transmitter and must be capable of handling the output power of the transmitter. Also, the duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver. Since duplexers are not available which operate at a separation of less than 24 MHz, the frequencies for each CPE must be 24 MHz apart and each CPE requires a different unique duplexer.
The system (CPE) of FIG. 1 transmits, illustratively, at 713 MHz and receives at 743 MHz. At the CPE the transmit power of 25 dBm is much greater than the received power of −92 dBm.
Using the full duplex DOCSIS system for wireless connectivity at a customer location, it was found necessary to be able to transmit a signal that is 0.5 watts (or 500 mwatts or 76 dBmV into a 75 Ohms antenna and also receive a signal that is 1.000e-12 watts (or 1.000e-9 mwatts or −40 dBmV) on the same antenna. As can be seen the transmit signal is huge compared to the receive signal. To be able to use the same antenna for transmit and receive signals, it was necessary to have a duplexer that can separate the two signals and achieve a reasonable level of signal to noise ratio on the receive signal. The duplexer has to be able to provide greater than 110 dB of separation between the two signals. Making a duplexer that can achieve 110 dB of separation is extremely difficult. Only fixed frequency duplexers can be made with fixed frequency components. If the transmit and receive frequencies are close together (less than 18 MHz), it is next to impossible to make a duplexer that can achieve 110 dB of separation. Ceramic duplexers are the only ones that are of reasonable size and they have been used to-date to provide the separation. Each time a new pair of frequencies is used, a new ceramic duplexer has to be custom built. There is no easy way of creating a frequency agile duplexer or equivalent. Also the ceramic duplexers have not been able to achieve the separation needed and the receive signals are corrupted by the transmit signals. Much higher receive signals are needed to reduce the corruption of the receive signals by the transmit signals.
The CPE equipment has two limitations, which limit the area over which satisfactory system operation can occur. First, the duplexer has difficulties separating the transmit and receive channels from one another. The second limitation is due to the fact that the gain of the common antenna does not have to be the same for both the transmit and the receive signals. The problem thus is to eliminate the need for a different duplexer for each CPE or provide a frequency agile duplexer.